Using unsaturated elastomers as substrates in olefin metathesis has been investigated for many years, with applications including molecular weight modification, microstructure elucidation via conversion to small molecules more amenable to characterization, the preparation of end-functionalized polymers and the recycling of commercial elastomers to chemical feedstock. A common feature of all resultant polymers reacted via metathesis is the unsaturation in the main chain, which limits their applications by the susceptibility of the carboncarbon double bonds to oxidative and thermal degradation. Therefore, hydrogenation of metathesed polymers is important to widen their range of applications.
Tandem metathesis/hydrogenation found an early application in the production of a polymer by the ring-opening polymerization of cyclic olefins. It allows the synthesis of lightweight, moldable polymers with desirable optical characteristics. During the tandem metathesis/hydrogenation process, the metathesed polymers are typically prepared, isolated, and purified prior to hydrogenation with additional reagents and/or catalysts. In U.S. Pat. No. 5,905,129 a binary catalytic system as e.g. WCl6/SntBu4 is used to catalyse ROMP of cyclic olefins and another catalyst is used for subsequent hydrogenation without the need of isolation of the polymer from the first step. In U.S. Pat. No. 6,197,894 a one-pot process of Mo-catalyzed ROMP, followed by homogeneous hydrogenation with RuCl2(PPh3)4 under forcing conditions (165° C., above 70 atm H2) has been described.
Homogeneous transition-metal catalysts are usually designed for a single reaction. Nowadays, the increasing demand for advantageous and efficient synthetic processes requires the development of tandem catalysis, in which one catalyst could be multi-functionalised, with two, or more, mechanistically distinct reactions being accomplished by single catalyst.
In the last decade, ruthenium carbene complexes have found extensive use in olefin metathesis, as for example reported by T. M. Trnka and R. H. Grubbs in Acc. Chem. Res. 2001, 34, 18. Meanwhile, these catalysts also showed to be effective catalysts for hydrogenation reactions. Tandem metathesishydrogenation processes using single ruthenium carbene complexes of the Grubbs-type RuCl2(═CHR)(PR′3)2 as catalysts have been developed. For instance, McLain et al. have reported in Proc. Am. Chem. Soc.; Div. Polym. Mater. Sci. Eng. 1997, 76, 246 the synthesis of an ethylene/methylacrylate copolymer by the ROMP of an ester-functionalized cyclooctene using RuCl2(═CHCl═CPh2)(PCy3)2, followed by the application of hydrogenation to completed ROMP reaction at 135° C. Dixneuf et al. in Green Chemistry, 2009, 11, 152 synthesized nitrile acid derivatives and alcohols by tandem metathesis hydrogenation of ester and acrylonitrile and cross metathesis hydrogenation of aldehydes.
Although some scientific research of synthetic rubber metathesis has been reported in the literature, such reports have mainly concentrated on metathetic degradation only. For example, Dimitry F. Kutepov et al. disclosed in J. Mol. Catal. 1982, 15, 207 the metathetic cyclodegradation of cis-polyisoprene to low molecular weight oligomers, and the co-metathesis of cis-polyisoprene with linear olefins to yield linear cis-oligomers was carried out using W[OCH(CH2Cl)2]2Cl4—AlEt2Cl-anisole as a catalyst.
W. B. Wagener et al. in Macromolecules 2000, 33, 1494 reported that the well defined ruthenium catalyst, Cl2(Cy3P)2Ru(═CHPh), effected the clean metathesis depolymerization of high molecular weight solid 1,4-polybutadiene at room temperature.
J-F. Pilard et al. in Macromol. Chem. Phys., 2005, 206, 1057 reported degradation studies of cis-1,4-polyisoprene using first and second generation Grubbs catalysts to achieve end-functionalized acetoxy oligomers in both an organic solvent and a latex phase at room temperature. Well-defined acetoxy telechelic polyisoprene structures were obtained in a selective manner with a range of Mn from 10,000 to 30,000, with a polydispersity index of around 2.5.
In WO2002/100941 A1 a process for preparing a hydrogenated nitrile rubber in an organic solvent is disclosed. In one embodiment of the reaction a nitrite rubber is subjected to metathetic degradation in a first step. Such metathesis is performed in organic solution using a ruthenium- or osmium based complex catalyst containing at least one carbene ligand like e.g. Grubbs II catalyst to achieve the decrease of the molecular weight of the initial nitrile rubber. Then, in a second step, however, without isolating the degraded nitrite rubber the reaction mixture is treated with hydrogen. In the presence of hydrogen Grubbs II catalyst, is converted to a dihydride complex (PR3)2RuCl2H2, which itself acts as an olefin hydrogenation catalyst and provides a high degree of hydrogenation. Thus, in a one-pot reaction a low molecular hydrogenated nitrile rubber can be obtained. However, this method occurred in an organic solution of the nitrile rubber. As the preparation of nitrile rubber is mostly conducted by water based emulsion polymerisation such type of reaction requires the isolation of the nitrile rubber after polymerisation prior to subjecting it to metathesis and hydrogenation which decreases the commercial attractiveness of the whole process.
WO 2005/080456 A describes the preparation of a hydrogenated nitrile rubber by hydrogenating nitrile rubber with a simultaneously occurring metathetic degradation of the nitrile rubber. Such process is conducted in the presence of a ruthenium- or osmium based complex catalyst containing at least one carbene ligand like e.g. Grubbs II catalyst and in an organic solvent, too. This process therefore suffers the same drawback as the process of WO2002/100941 A1 with regard to the necessary isolation of the nitrile rubber after the polymerisation reaction. Additionally it may be difficult to control two different activities, i.e. the metathetic and the hydrogenation activity of the catalyst and therefore to obtain hydrogenated nitrile rubber with reproducible molecular weights and hydrogenation degree.
The present invention therefore has the object to provide a process allowing combining the metathetic degradation and hydrogenation of a diene-based polymer present in an aqueous suspension, i.e. as a latex, to obtain a hydrogenated nitrile rubber with a lowered molecular weight and a high degree of hydrogenation within acceptable short reaction times.